Significant advancement of office automation equipment in recent years has helped to promote the widespread use of image forming apparatuses, such as a copier, a printer and a facsimile machine, which apparatuses perform an image forming process by means of an electrophotographic printing method. Such apparatuses that employ the electrophotographic printing method generally perform a charging step, an exposure step, a development step, a transfer step, a cleaning step and a fixing step so that an image is formed. Specifically, in the charging step, the surface of a photoreceptor, i.e. an image bearing member, is uniformly charged in darkness. In the exposure step, projection of signal light corresponding to an image to be formed on the charged photoreceptor removes the charge of the portion exposed, so that an electrostatic image (an electrostatic latent image) is formed on the surface of the photoreceptor. In the development step, the electrostatic image on the surface of the photoreceptor is supplied with a toner for developing an electrostatic image (which toner is hereinafter referred to as “toner” unless otherwise indicated) so that a toner image (visible image) is formed. In the transfer step, a recording material such as paper and sheet is allowed to contact the toner image formed on the surface of the photoreceptor. In addition, corona discharge is performed on the side of the recording material which side is opposite from the side that contacts the toner image so that charge having a polarity opposite the polarity of the toner is provided for the recording material. This allows the toner image to be transferred onto the recording material. In the fixing step, the toner image on the recording material is fixed by heating, pressing, and/or other means. In the cleaning step, toner which remains on the surface of the photoreceptor without being transferred onto the recording material is collected. Through the above steps, image forming apparatuses which employ the electrophotographic printing method form a desired image on recording material.
Image forming apparatuses which employ the electrophotographic printing method use, as a developer for developing a toner image, either: a one component developer which consists of a toner alone; or a two component developer which consists of a toner and a carrier. The inclusion of a carrier provides a two component developer the functions of stirring, carrying and charging toner particles. Since the toner contained in a two component developer does not need to have the functions of the carrier, the two component developer has the advantages of better controllability due to such separation of the functions and of allowing for a high quality image, in comparison with a one component developer, which is formed solely from toner particles. Accordingly, a lot of research and development has been conducted in connection with a toner suitable for use in combination with a carrier.
The carrier has two fundamental functions: the function of stably charging a toner to a desired level; and the function of carrying the toner to the photoreceptor. Further, the carrier is stirred in a developing tank and carried onto a magnet roller, on which the carrier forms a magnetic brush. Subsequently, the carrier passes through a regulating blade and then returns to the inside of the developing tank. This allows the carrier to be reused. The carrier is required, although reused, to stably realize the fundamental functions: in particular, the function of stably charging a toner. Some methods have been proposed for coating the surface of a carrier with a resin for the purpose of maintaining such a function of the carrier.
Such methods include, for instance, one for coating the surface of a carrier with a styrene-acrylic copolymer resin or a polyurethane resin, each of which has high, and another method for coating the surface of a carrier with a fluorine resin, which conversely has low surface tension. Although a resin having high surface tension has great adhesiveness to a carrier core, the resin has the problem of toner-spent. On the other hand, a fluorine resin having low surface tension has poor adhesiveness to a carrier core although being effective against the toner-spent problem. Therefore, when the carrier coated with such a fluorine having high surface tension is stirred in the developing tank, the resin coating separates from the carrier core. This inhibits stable charging.
For the purpose of solving the above problems, a carrier coated with a silicone resin has been proposed. However, a desired charge amount is unobtainable by solely providing a silicone resin coating. In light of this, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 284862/1989 (Tokukaihei 1-284862; published on Nov. 16, 1989)), for example, discloses a method for adding a silicone coupling agent having an amino group to a resin.
Further, Patent Document 2 (Japanese Unexamined Patent Application Publication No. 29859/1989 (Tokukaishou 64-29859; published on Jan. 31, 1989)), for example, discloses a carrier whose surface is mechanically coated with a charge control agent so that the toner is stably charged.
Still further, Patent Document 3 (Japanese Unexamined Patent Application Publication No. 204666/1992 (Tokukaihei 4-204666; published on Jul. 27, 1992)), for example, discloses a developer which includes a carrier whose coating includes: a particular toner external additive; and quaternary ammonium salt as a charge control agent. The carrier described in Patent Document 3 is treated with toner external additives, i.e. a positively charged silicone coupling agent and a negatively charged silicone coupling agent. In addition, the carrier includes in the coating thereof quaternary ammonium salt. According to Patent Document 3, the use of such a carrier can suppress variations in charging of the toner which variations are caused by the environmental conditions.
Patent Document 4 (Japanese Unexamined Patent Application Publication No. 11864/1987 (Tokukaishou 62-11864; published on Jan. 20, 1987)), for example, discloses a developer including a carrier coated with a resin which contains a charge control agent of the same polarity as the polarity of the toner. According to Patent Document 4, any resin may be used for the coating of the carrier, and the inclusion of a charge control agent in the resin is characteristic.
Further, Patent Document 5 (Japanese Examined Patent Publication No. 49982/1993 (Tokukouhei 5-49982; published on Jul. 27, 1993)), for example, discloses a developer which includes a carrier coated with a positively or negatively charged synthetic resin, polarity of which is the same as the polarity of the toner.